Gas-assisted rapid thermal processing

ABSTRACT

A system, method and apparatus for processing a semiconductor device including a processing chamber and a heating assembly positioned within the processing chamber. The heating assembly including at least a plate defining an internal cavity configured to receive gas. The gas enters the internal cavity through a first passage at a first temperature, and exits the internal cavity at a second temperature through a second passage.

BACKGROUND

1. Field of the Invention

This invention generally relates to semiconductor manufacturingequipment and, more particularly, to an improved method for rapidthermal processing of a semiconductor wafer.

2. Related Art

In the semiconductor manufacturing industry, depending upon theparticular process, a semiconductor wafer may be treated at temperaturesof from about 100° C. to about 1300° C., under controlled conditions, inrather sophisticated furnaces. Commonly, these furnaces are horizontalor vertical type furnaces, which use various energy sources to heat thewafer, including radiant heaters, arc lamps, and tungsten-halogen lamps.As shown in FIG. 1, a typical horizontal or vertical type furnacerequires a time t₁ for ramping up to a particular process temperature toprocess the wafers. The ramp up rate for a typical furnace is usuallybetween 5° C./min to about 15° C./min, which makes time t₁ typically onthe order of about 1 hour. A time t₂ is required for cooling of thewafers, which is generally on the order of about 2 hours. Longprocessing times are typically unacceptable in advanced semiconductordevice manufacturing because of dopant redistribution, excessive costs,excessive exposure to temperature, and high power requirements.

In order to continue to make advancements in the development ofsemiconductor devices, especially semiconductor devices of decreaseddimensions, new processing and manufacturing techniques have beendeveloped. One such processing technique is know as Rapid ThermalProcessing (RTP), which reduces the amount of time that a semiconductordevice is exposed to high temperatures during processing. The rapidthermal processing technique, typically includes raising the temperatureof the wafer and holding it at that temperature for a time long enoughto successfully perform a fabrication process, and avoid such problemsas unwanted dopant diffusion that would otherwise occur at the highprocessing temperatures.

SUMMARY

The present invention provides an improved system, method and structurefor rapid thermal processing of a semiconductor wafer. The presentinvention helps to improve process times so as to avoid, for example,the creation of thermal gradients in the process chamber, which cancause slip and warpage of the wafer. The present invention adds asignificant conductive heat transfer component to processes which maypresently use primarily radiant heat transfer to raise the temperatureof a semiconductor wafer.

In accordance with the present invention, a gas, such as He, H₂, O₂, Ar,N₂, and gases containing He, H₂, O₂, Ar and N₂, can be introduced intothe processing chamber during processing. The gas is introduced throughat least one heatable member, such as a heatable plate that can overlaya surface of the wafer. The heatable member includes an internal cavityin which heat is transferred to the gas. The heated gas exits theheatable member in the direction of the wafer surface through an outletportion of the plate. The outlet portion can include a plurality ofholes, which are dispersed on the plate surface so as to evenlydistribute the heated gas across the surface of the wafer.Advantageously, the addition of the heated gas to the process can raisethe temperature of the wafer much more quickly then without the use ofthe gas.

These and other features and advantages of the present invention will bemore readily apparent from the detailed description of the preferredembodiments set forth below taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a graph representative of typical temperature heat-up andcool-down rates for a conventional wafer processing furnace;

FIG. 2 illustrates schematically a side view of one embodiment of asemiconductor wafer processing system in accordance with the presentinvention;

FIG. 3A is a simplified cross-sectional illustration of an embodiment ofa processing chamber in accordance with the present invention;

FIG. 3B is a simplified illustration of a side view of a heatingassembly in accordance with the present invention;

FIG. 4A is a top view of a heatable plate including standoffs forholding a wafer in accordance with an embodiment of the presentinvention;

FIG. 4B is a cross-sectional view of the heatable plate of FIG. 4A;

FIG. 5 is a simplified cross-sectional illustration of an embodimentwith directional gas flow indications in accordance with the presentinvention;

FIG. 6 is a graph comparing wafer temperature as a function of timebetween a wafer disposed in the process chamber of FIG. 3 without gasflow and a wafer disposed in the process chamber of FIG. 3 with gas flowin accordance with principles of the present invention;

FIG. 7 illustrates an alternative embodiment of the process chamber ofFIG. 3 used for cooling a wafer; and

FIG. 8 illustrates yet another alternative embodiment of the processchamber of FIG. 3 used for cooling a wafer.

DETAILED DESCRIPTION

FIG. 2 illustrates schematically a side view of one embodiment of asemiconductor wafer processing system 10 that establishes arepresentative environment of the present invention. Processing system10 includes a loading station 12, a transfer chamber 20 and a reactor26. A robot 22 within transfer chamber 20 rotates toward loadlock 18 andpicks up a wafer 24 from cassette 16. Reactor 26, which may also be atatmospheric pressure or under vacuum pressure, accepts wafer 24 fromrobot 22. Robot 22 then retracts its arm which carried wafer 24 and,subsequently, the processing of wafer 24 begins.

Reactor 26 may be any type of reactor which allows wafers to be loadedat wafer processing temperatures, of between about 100° C. to about1300° C., without adverse results. In accordance with the presentinvention, the reactor may be a hot-walled RTP reactor, such as is usedin thermal anneals. In other embodiments, the reactor may be the type ofreactor used for dopant diffusion, thermal oxidation, nitridation,chemical vapor deposition, and/or similar processes. In one embodiment,process gases, coolant, and electrical connections may be providedthrough the rear end of the reactors using interfaces 34.

FIG. 3A is a simplified cross-sectional illustration of an embodiment ofa reactor 100 in accordance with the present invention. Reactor 100 maygenerally include a closed-end process chamber 106, which defines aninterior cavity 108. Process chamber 106 may be constructed with asubstantially rectangular cross-section, having a minimal internalvolume surrounding wafer 110. In one embodiment, the volume of processchamber 106 may be no greater than about 5000 cm³, preferably less thanabout 3000 cm³. One result of the small volume is that uniformity intemperature is more easily maintained. Additionally, the small processchamber volume allows reactor 100 to be made smaller, and as a result,processing system 10 may be made smaller, requiring less clean roomfloor space. Process chamber 106 may be made of aluminum, quartz orother suitable material, such as silicon carbide or Al₂O₃. To conduct aprocess, process chamber 106 should be capable of being pressurized.Typically, process chamber 106 should be able to withstand internalpressures of about 0.001 Torr to 1000 Torr, preferably between about 0.1Torr and about 760 Torr.

Opening 104, shown at the left end of process chamber 106, providesaccess for the loading and unloading of wafer 110 before and afterprocessing. Opening 104 may be a relatively small opening, but with aheight and width large enough to accommodate a wafer of between about0.5 to 2 mm thick and up to about 300 mm (˜12 in.) in diameter, and thearm of robot 22 passing therethrough. The height of opening 104 is nogreater than between about 18 mm and 50 mm, and preferably, no greaterthan 20 mm. The relatively small opening size helps to reduce radiationheat loss from process chamber 106, and keeps down the number ofparticles entering cavity 108 to allow for easier maintenance of theisothermal temperature environment.

In one embodiment, a plurality of heating elements 114 surround a topand a bottom portion of process chamber 106. Resistive heating elements114 may be disposed in parallel across chamber 106 such that eachelement 114 is in relative close proximity to each other element 114.For example, each resistive heating element 114 may be spaced betweenabout 5 mm and 50 mm apart; preferably between about 10 mm and 20 mmapart. Accordingly, the close spacing of heating elements 114 providesfor an even heating temperature distribution across the wafer positionedin cavity 108. Resistive heating element 114 may include a resistiveheating element core and a filament wire. The core is usually made of aceramic material, but may be made of any high temperature rated,non-conductive material. The filament wire is conventionally wrappedaround the core to allow an optimal amount of heat energy to radiatefrom the element. The filament wire may be any suitable resistivelyheatable wire, which is made from a high mass material for increasedthermal response and high temperature stability, such as SiC, SiC coatedgraphite, graphite, NiCr, AlNi and other alloys. Preferably, theresistive heating filament wire is made of a combination Al—Ni—Fematerial, known commonly as Kantal A-1 or AF, available from Omega Corp.of Stamford, Conn.

A direct line voltage of between about 100 volts and about 500 volts maybe used to power the resistive elements. Thus, in this embodiment nocomplex power transformer is needed for controlling the output ofresistive heating elements 114.

In one embodiment, reactor 100 includes heat diffusing members 118 and120, which are positioned proximate to and typically overlay heatingelements 114. Heat diffusing members 118 and 120 absorb the thermalenergy output from heating elements 114 and dissipate the heat evenlywithin chamber 106. It should be appreciated that by heating wafer 110from above and below, and further by keeping the distance between heatdiffusing members 118 and 120 small, the temperature gradient withinchamber 106 is more easily isothermally maintained. Heat diffusingmembers 118 and 120 may be any suitable heat diffusing material that hasa sufficiently high thermal conductivity, preferably Silicon Carbide,Al₂O₃, or graphite.

As further illustrated in FIGS. 3A and 3B, in one embodiment, reactor100 includes a heating assembly 122. Heating assembly 122 includes twoheatable members or top plate 124 and bottom plate 126 positioned, asshown in FIG. 3, adjacent and opposed to one another. Top plate 124 isspaced apart from bottom plate 126 a distance φ which allows wafer 110to be placed therebetween. For example, the distance φ between plates124 and 126 can be between about 10 mm to about 50 mm.

Heating plates 124 and 126 of heating assembly 122 may have a large massrelative to wafer 110, and may be fabricated from a material, such assilicon carbide, quartz, Inconel, aluminum, steel, or any other materialthat does not react at processing temperatures with any ambient gases orwith wafer 110.

Arranged on a top surface of bottom plate 126 may be wafer supports 112.In one embodiment, wafer supports 112 extend outward from the surface ofbottom plate 126 to support wafer 110. Wafer supports 112 are sized toensure that wafer 110 is held in close proximity to bottom plate 126.For example, wafer supports 112 may each have a height of between about50 μm and about 20 mm, preferably about 2 mm to about 8 mm. The presentinvention includes at least three wafer supports 112 to ensurestability. However, the total area of contact between wafer supports 112and wafer 110 is less than the size of the wafer.

Top and bottom plates 124 and 126 may be formed into any geometricshape, preferably a shape which resembles that of the wafer. In oneembodiment, plates 124 and 126 are circular plates. The dimensions ofplates 124 and 126 may be larger than the dimensions of wafer 110, suchthat the surface area of wafer 110 can be completely overlaid by thesurface area of heating plates 124 and 126. In one example, the circulardiameters of heating plates 124 and 126 is greater than the diameter ofwafer 110. For example, as illustrated in FIG. 4A, the radius of bottomheating plate 126 is greater than the radius of wafer 110 by about alength γ, which can range between about 1 mm and 100 mm, preferably25-to-50 mm.

For convenience of understanding, FIG. 4A illustrates an embodiment of aheatable member, for example, top plate 124. It should be understoodthat top plate 124 is similar in structure and function to bottom plate126, and the description of top plate 124 is the same as for bottomplate 126, unless otherwise noted. In accordance with one embodiment ofthe present invention, on a periphery of heating top plate 124 is atleast one heat source 222. Heat source 222 may be a resistive heatingelement or other conductive heat source, which can be made to contact aperipheral portion of top heating plate 124 or may be embedded withintop heating plate 124. The resistive heating element may be made of anyhigh temperature rated material, such as a suitable resistively heatablewire, which is made from a high mass material for increased thermalresponse and high temperature stability, such as SiC, SiC coatedgraphite, graphite, AlCr, AlNi and other alloys. Resistive heatingelements of this type are available from Omega Corp. of Stamford, Conn.Alternatively, top heating plate 124 can be heated using a radiantsource.

In one embodiment, plates 124 and 126 may be positioned suspended withinprocess chamber 106, in a cantilevered relationship on a wall of theprocess chamber. In this embodiment, coupling mechanism 224 includes amounting bracket 228 and electrical leads 230 to provide an electricalpower connection to heat source 222. Mounting bracket 228 can be coupledto an internal wall of process chamber 106 using conventional mountingtechniques. Once mounted, electrical leads 230 can extend outside ofprocess chamber 106 to be connected to an appropriate power source. Thepower source may be a direct line voltage of between about 100 volts andabout 500 volts. Alternatively, top plate 124 may hang suspended frommounts emanating down from a ceiling of process chamber 106 and bottomplate 126 may rest on mounts emanating up from a floor of processchamber 106.

FIG. 4B is a simplified cross sectional view of heating plate 124, 126,in accordance with the present invention. Heating plates 124, 126 caninclude a hollowed out portion defining an internal cavity 250. Internalcavity 250 has an inlet 252, which enters internal cavity 250 on a firstside 256 of plate 124, 126. Inlet 252 allows a gas to be fed from a gasreservoir (not shown) into internal cavity 250. The gas may include, forexample, any suitable gas, such as He, H₂, O₂, Ar, N₂ and the like.

Internal cavity 250 has an outlet portion 254, which is defined on asecond side 258 of plate 124, 126. Outlet portion 254 allows the gasthat has entered internal cavity 250 to escape the cavity. In oneembodiment, outlet portion 254 can be defined as a plurality of outletsor holes 255. Holes 255 can be any suitable size that allow passage ofthe gas. In one example, holes 255 may be between about 0.1 mm to about2 mm in diameter.

In one embodiment, internal cavity 250 may include a baffle 260,positioned between inlet 252 and outlets 255. In this embodiment, baffle260 impedes the flow of the gas entering through inlet 252. By impedingthe otherwise forced gas flow before exiting through outlets 255, thegas is made to reside in internal cavity 250 for a longer time period,which allows more heat to be transferred to the gas before the gas exitsthe cavity.

In one embodiment, the thickness of top and bottom plates 124 and 126 isany thickness d suitable for accommodating heat source 222 and forproviding an adequately sized internal cavity 250. The thickness d canrange from between 1 cm and 10 cm; preferably about 3 cm. In oneexample, plates 124 and 126 can be fabricated using a splitconstruction, which means that each plate is originally two pieces 263and 267, joined at seam 265, for example, by welding. Baffle 260 can bemounted onto piece 267, for example, using attaching device 261, such asscrews, nuts and bolts, rivets and the like and a spacer 269. It shouldbe noted that one of ordinary skill in the art may fabricate plates 124and 126 using a variety of well known techniques, all of which fallwithin the scope of the present invention. Generally, plates 124 and 126may be made of materials, such as Al, and Al alloys Ni, Inconel, Mo,stainless steel, SiC and the like.

Referring now to FIGS. 4B, 5 and 6, in operation, internal cavity 250serves as a heat exchanger, such that the gas can be heated as ittravels from inlet 252 through to the exit points of outlets 254. Theentering gas can be at ambient temperature or may be pre-heated prior toentering internal cavity 250. The gas is made to move through internalcavity 250 which may be heated by heat source 222 to betweenapproximately 100° C. and 1000° C. It should be understood that thetemperature of the gas exiting internal cavity 250 depends on, forexample, the type of gas, the flow rate of the gas, the residence timeof the gas in internal cavity 250 and the nominal temperature ofinternal cavity 250. Each of these parameters can be adjusted until theexiting gas temperature is appropriate for a specific process.

As shown in FIG. 5, in one embodiment, wafer 110 is placed between topand bottom plates 124 and 126 and a gas, such as N₂, is allowed to flowthrough internal cavity 250 of top plate 124. In this example, the gasenters internal cavity 250 at approximately room temperature (25° C.).The gas is heated by radiation and conduction as it passes throughcavity 250 and exits outlets 255.

FIG. 6 is a graph showing an exemplary result of a heating process usingthe configuration of FIG. 5. In this example, heat source 222 is set toabout 220° C., which causes the gas to exit outlets 255 at approximately215° C.

As illustrated by line 264 in FIG. 6, allowing gas to flow through topplate 124 only, wafer 110 reaches the temperature of approximately 220°C. in about 60 seconds. Line 266 illustrates approximately the sameheating profile if gas is allowed to simultaneously flow through bottomplate 126. For comparison, line 262 represents the heating profile ofwafer 110 without gas flow through either top plate 124 or bottom plate126. As illustrated by line 262, approximately 270 seconds are requiredto first heat wafer 110 to approximately 210° C.

FIG. 7 is a simplified illustration of another embodiment in accordancewith the present invention. In this embodiment, process chamber 106 isstructurally the same as in other embodiments, including top plate 124and bottom plate 126. However, instead of heating the gas as it enterschamber 106, gas is removed from chamber 106, through top plate 124 andbottom plate 126 to provide forced convection cooling of wafer 110. Inoperation, when cooling is desired, a vacuum is pulled through inlets252 to cause gas to flow out from chamber 106 in a direction generallyindicated by air flow arrows 270 and into outlets 255. In thisembodiment, outlet portion 254 becomes an inlet to internal cavity 250and gas inlet 252 provides an outlet for internal cavity 250.

In one embodiment, an exhaust port 272 in chamber 106 can be opened toprovide ambient air for cooling. As the ambient air is moved over thesurface of wafer 110, the wafer surface is cooled. In one embodiment,the cooling rate can range from between approximately 5° C./sec and 20°C./s.

FIG. 8 shows another embodiment for moving gas over the surface of wafer110. In this embodiment, a gas inlet 280 is positioned on chamber 106approximately in-line and parallel to wafer 110. An outlet or exhaustport 282 is also positioned in-line and parallel to wafer 110, andapproximately in-line with inlet 280. In operation as a gas is allowedinto chamber 106 through inlet 280, outlet port 282 , under vacuum,causes the gas to flow over wafer 110 providing a force convectioncooling effect.

Having thus described embodiments of the present invention, personsskilled in the art will recognize that changes may be made in form anddetail without departing from the spirit and scope of the invention.Thus the invention is limited only by the following claims.

1. A system for processing a semiconductor device, the systemcomprising: a processing chamber heated by heating elements; a firstplate positioned within said processing chamber and defining a firstinternal cavity configured to receive a first gas through a firstpassage into said first internal cavity at a first temperature and toemit said first gas from said first internal cavity at a secondtemperature through a second passage; and a second plate disposedadjacent to said first plate, said first plate and said second plateprovided between said heating elements and defining a processing areatherebetween, said second plate defining a second internal cavityconfigured to receive a second gas through a first passage into saidsecond internal cavity at a first temperature and to emit said secondgas from said second internal cavity at a second temperature through asecond passage, said first emitted gas and said second emitted gasvarying the temperature of said processing area.
 2. The system of claim1, wherein said second passage comprises a plurality of holes defined ona surface of said first and said second plates.
 3. The system of claim1, wherein said first plate and said second plate comprise a heat sourcefor heating said plate to a preselected temperature.
 4. The system ofclaim 1, wherein said first gas is taken from the group consisting ofN₂, He, H₂, O₂, Ar and gas mixtures containing He, H₂, O₂, Ar and N₂. 5.The system of claim 1, wherein said internal cavity further comprises abuffer to disperse said first gas throughout said internal cavity.
 6. Asystem for water processing comprising: a chamber heated by heatingelements; and a first heatable plate and a second heatable platepositioned between said heating elements within said chamber, anddefining a processing area therebetween, each of said heatable platesincluding: an internal cavity defining an internal wall and configuredto receive a gas; means for heating said internal wall to a preselectedtemperature; and an outlet portion defining a plurality of holes foremitting said gas to said processing area; said gas varying thetemperature of said processing area.
 7. The system of claim 6, whereinsaid gas is taken from the group consisting of He, H₂, O₂, Ar, N₂ andgas mixtures containing He, H₂, O₂, Ar, and N₂.
 8. The system of claim6, wherein said internal cavity further comprises a buffer to dispersesaid first gas throughout said internal cavity.